Viewing windows of this kind are generally known. They are, as a rule, mounted in the front end door of a baking oven or the like. The following description is directed to household cooking ovens. The invention is, however, applicable to any other apparatus that has a viewing window or viewing door which closes off the interior space or cavity wherein the temperature deviates from the ambient temperature of the apparatus. The viewing window or viewing door permits looking into this interior cavity. Accordingly, the viewing window or viewing door is also applicable to freezers and refrigerators or dryers.
In viewing windows for ovens, the inner viewing plate and possible additional viewing plates as well as the front plate are attached to the sheet metal of the door. The viewing plates are inter alia heated by heat conduction because the sheet metal of the door is in direct contact with the cooking cavity and is made of a material (metal) which is a good heat conductor.
In the case of viewing windows, it is, however, in contrast to the remaining walls, which surround a temperature-elevated interior cavity, not possible to provide an effective heat insulation such as with mineral wool and the like. For this reason, it is with precisely such viewing windows that the problem is present that the outer surfaces thereof become too hot.
To reduce the outside temperature of the viewing windows, arrangements are generally used which include multiple plates having an outer plate which does not border directly on the temperature-elevated cavity so that heating of the outside plate by direct heat transfer is avoided. However, even for multiple mutually spaced plates with an intermediate air space, heating of the outside plate by radiation from the temperature-elevated interior cavity occurs. This radiation passes the inner plates, in part, and is directly absorbed by the outer plate and partially also by the inner plates, which, in turn, then transmit heat radiation to the next outer plate or via an air-filled intermediate space to the next outer plate by heat conduction and convection.
Embodiments of viewing windows are already known wherein an outer plate is held forward of the remaining plates at a spacing in such a manner that the frame of the arrangement is open at least at the top and bottom between the outer plate and the next inner plate so that an air convection in the ambient space outside of the apparatus can take place between this outer plate and the next inner plate. On the one hand, the air which becomes heated between the two outer plates is exchanged with the ambient air by circulation and, on the other hand, the outer plate is continuously cooled by this air flow.
German patent publication 7,706,648 discloses a transparent heat-protective unit for cooking ovens, dryers and the like. The heat protective unit has at least two parallelly arranged glass plates and an edge joiner extending completely around the periphery. The edge joiner connects the glass plates to each other and holds the same at a pregiven spacing from each other. The edge connector has a spacer section which is placed between the peripheries of the plates and each glass plate is cemented by means of a sealing mass to each other and to this spacer profile. The sealing mass remains elastic even under the influence of heat.
It is an object of the invention of German patent publication 7,706,648 to provide a heat-protective unit wherein the thermal insulation is improved and to prevent the inner side thereof from becoming vapor coated and dirtied.
German patent publication 3,602,455 discloses a cooking oven for baking, roasting, grilling and the like and having a cooking cavity with at least one heating element and a door closing off the cooking cavity. The heating element is provided on the upper side of the cooking cavity and the door comprises two parallel door shells mounted at a spacing from each other. Each of the door shells has a glass plate and the two door shells are connected to each other by rails arranged therebetween. The rails each have parallel legs which run spaced from each other. The rails have U-shaped or Z-shaped cross sections and the outer legs thereof are cemented to the outer door shell.
German patent publication 3,602,455 discloses a cooking oven which is so configured that the outer door shell has no regions at which dirt can collect and an excessive heat-up of the outer door shell is avoided.
It is also possible to utilize thermochrome plates in the interior having radiation penetrability which is reduced with a temperature increase. This, however, affects viewing through the window.
It is also known to provide one or more plates of a multi-plate arrangement with a coating which either reflects infrared radiation or reduces the emission of infrared radiation. A coating having both these characteristics is, for example, a coating of fluorine-doped tin dioxide. If the coating is applied to the inner side of a plate facing toward the temperature-elevated cavity, then the coating reflects a portion of the infrared radiation back into the interior cavity. If the coating is applied to the outer side of a plate, then the coating reduces the infrared radiation in the outward direction to the next outer plate. The plates can also be coated on both sides.
German patent publication 4,206,820 discloses a cooking oven door for a cooking oven having a viewing window of which at least one glass plate has a heat-radiation reflecting coating. The reflecting coating is applied to the surface of the glass plate facing away from the interior oven cavity.
It is also an object of this invention to effect a reduction of the temperature at the front side of the cooking oven.
Notwithstanding the above-mentioned measures, it occurs frequently that, in household cooking ovens, the temperature of the outer plate of a viewing window rises up to 150.degree. C. and higher. This temperature rise occurs especially in household cooking ovens which are operated at temperatures of 450.degree. C. to 500.degree. C. and more, especially during self-cleaning utilizing pyrolysis. To make the viewing window safe with respect to contact therewith by an operator, it is a goal to limit the permissible outside temperature of such viewing windows to 78.degree. C. which is consistent with common standards (see, for example, UL 858, Underwriter Laboratory Household Domestic Appliances).
Furthermore, up to now, each plate is individually fastened during the assembly of the viewing plates by the oven manufacturer. This practice is associated with high cost with respect to both materials and time.